I'll probably eventually get one (unless a 24-105 II comes out that is even better!) but it is low on my priority list of lenses to get. Certainly not before I evaluated the 200-400 extender whenever that's out.
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 15-85, TS-E 24, MP-E 65, Zeiss 50/2 macro, Sigma 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye